Proceedings Volume 5157

Optical Spectroscopic Techniques and Instrumentation for Atmospheric and Space Research V

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Proceedings Volume 5157

Optical Spectroscopic Techniques and Instrumentation for Atmospheric and Space Research V

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Volume Details

Date Published: 3 November 2003
Contents: 8 Sessions, 25 Papers, 0 Presentations
Conference: Optical Science and Technology, SPIE's 48th Annual Meeting 2003
Volume Number: 5157

Table of Contents

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Table of Contents

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  • Ground-based Measurements: Radiation, Aerosols, and Clouds
  • Airborne Sensors Used for Spaceborne Sensor Validation
  • Space-based Sensors: Early Performance of New Instruments and Expectations for Soon-to-be-Launched Systems
  • Radiative Transfer and Geophysical Information Retrieval
  • Instrument Prototype Development for Enabling Future Space-based Observation Capabilities
  • New Instrument Concepts for Future Space-based Remote Sensing Systems
  • Remote Sensing Technologies
  • Poster Session
Ground-based Measurements: Radiation, Aerosols, and Clouds
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Statistics of Alaskan clouds from the Infrared Cloud Imager (ICI)
Joseph A. Shaw, Brentha Thurairajah
The Infrared Cloud Imager (ICI) is a ground-based infrared thermal imaging system used to identify and classify clouds based on the downwelling atmospheric radiance in the 8-14 μm wavelength region. Data obtained during a deployment in Barrow, Alaska during February - May 2002 has been combined with radiosonde and microwave radiometer data to calculate spatial cloud statistics for the months of March and April 2002. The results show a general trend of increasing cloudiness during this period, and a short transition time between clear and cloudy skies. A comparison of cloud statistics computed from the full ICI images with statistics computed from a single ICI pixel suggest that a point sensor would underestimate cloudiness during this data period.
Remeasuring the extraterrestrial solar spectral irradiance
Lee Harrison, Jerry L. Berndt, Piotr W. Kiedron
We will measure the solar spectral irradiance by deploying a CCD-array solar spectrograph to a high altitude favorable site, as part of a self-contained autonomous system with a calibration system using a monochromator and absolute photodiode "trap detectors." Data will be reduced using Langley extrapolation (and in the stronger absorption bands methods similar to Reagan-Brugge fitting), to yield the solar output free of atmospheric absorption. This measurement system will substantially improve the accuracy of the field measurements by making the instrument continuously self-calibrating against a local absolute standard in the range 400 - 900 nm. In the ranges 360 - 400 nm and also 900 - 1100 the trap detectors are not an absolute standard, but serve as a very reproducible transfer standard from an irradiance scale to be taken from either NIST lamps, or more recently-introduced detectors with calibrated efficiencies. We expect an absolute accuracy of 0.3% for solar-spectrum determination in the range 400 to 900 nm, not including the O2 band at 760 nm, and the H2O bands at 820 and 940 nm. In the 360 - 400 nm domain we may be able to extend trap-detector quantum efficiency to allow an accuracy better than a secondary irradiance transfer, otherwise this domain and the range 900 to 1100 nm will have an accuracy of ≈ 1 %. The extrapolations in the strong-absorption bands will have an increased uncertainty which can be estimated from the statistics of the data. We describe the instrument and self-calibration methadologies and design.
Compiled aerosol optical depth and extinction-to-backscatter climatology for Tucson, AZ
Matthew E. Mavko, Kurtis J. Thome, John A. Reagan
The Aeronet network has had a wide-ranging impact on the study of atmospheric aerosols, both temporally and geographically. This paper examines the results of measurements from the Aeronet network from a radiometer deployed in Tucson, Arizona during 1999 and 2000. Monthly averages of aerosol optical depth and Angstrom parameter values are presented. These show that a maximum in aerosol loading occurs in summer months with an average value for optical thickness of 0.11 at 670 nm compared to 0.03 during winter months. The Angstrom coefficient shows a similar trend with largest values, corresponding to smaller-sized aerosols dominating during the summer months. These results show significant differences from results obtained from similar measurements during the period of 1975-77. In addition to optical depth, aerosol extinction-to-backscatter ratio, or lidar aerosol ratio, is calculated and examined using size distribution data available from Aeronet and Mie scatter computations. This ratio varies from an average value of near 25 in March, April, and May to values near 100 for October, November, and December. Comparison of a subset of these data to those from an independent solar radiometer support these conclusions.
Airborne Sensors Used for Spaceborne Sensor Validation
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Validation studies using NAST-I measurements from recent field campaigns
Allen M. Larar, William L. Smith, Daniel K. Zhou, et al.
The Integrated Program Office (IPO) developed and supports high-altitude aircraft flights of the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounding Testbed (NAST) as part of risk mitigation activities for future NPOESS sensors. The NAST-Interferometer (NAST-I) is a high spectral and spatial resolution (0.25 cm-1 and 0.13 km nadir footprint per km of aircraft altitude, respectively) cross-track scanning (2 km swath width per km of altitude) Fourier Transform Spectrometer (FTS) observing within the 3.7 - 15.5 micron spectral range. NAST-I infrared spectral radiances are used to characterize atmospheric thermal and moisture structure and provide information on radiatively active trace gases (e.g. O3 & CO) observed during flights. These direct and derived NAST-I data products greatly contribute toward instrument and forward model pre-launch specification optimization and will enhance post-launch calibration/validation activities for the Cross-track Infrared Sounder, CrIS, to fly on NPP and NPOESS (as well as for other advanced atmospheric spaceborne sensors). In this paper we address some of the challenges associated with validating infrared spectral radiances obtained from such high spectral resolution remote sensing systems. This will include comparison of NAST-I infrared spectral radiances measured during recent field experiment campaigns with other radiance measurements as well as radiance calculations performed using Line-by-Line (LBL) forward radiative transfer model based on independent, nearly-coincident observations of atmospheric state.
MATR thermal channel analysis and results for autumn 2001 campaign
Jianguo Niu, Merritt N. Deeter, Allan Hills, et al.
The MOPITT (Measurements of Pollution in the Troposphere) Airborne Test Radiometer (MATR) uses gas filter correlation radiometry from high altitude aircraft to measure tropospheric carbon monoxide. This is in support of the ongoing validation campaign for the MOPITT instrument on board the Tera Satellite. This paper reports on a recent study of MATR CO retrievals using observations of thermal radiation during the autumn of 2001 in western United States. Retrievals of CO were performed and compared to in-situ sampling with less than 7% retrieval error relative to the in-situ total column amount. The effects that influence the retrieval such as the instrument sensitivity, the retrieval sensitivity, and bias between observations and the radiation model are discussed.
Space-based Sensors: Early Performance of New Instruments and Expectations for Soon-to-be-Launched Systems
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First-year measurements of stratospheric aerosol and gas experiment III/meteor
William P Chu, Chip Trepte, Joe Zawodny, et al.
The Stratospheric Aerosol and Gas Experiment III/Meteor Instrument was launched from Baikonur, Kazakhstan on December 10, 2001. After initial commissioning phase activities, it began routine solar occultation measurements by March 2002. During the first year of operation, additional measurement capabilities such as lunar occultation and limb scattering were successfully implemented with the SAGE III instrument. This paper will present a summary of the various data sets gathered from the SAGE III instrument during the first year of operation. Measurements of ozone, aerosol, and nitrogen dioxide from solar occultation, lunar occultation, and limb scattering techniques will be presented and discussed.
Operational performance of the TIMED Doppler Interferometer (TIDI)
Wilbert R. Skinner, Rick J. Niciejewski, Timothy L. Killeen, et al.
The TIMED Doppler Interferometer (TIDI) is a Fabry-Perot interferometer designed to measure winds in the mesosphere and thermosphere (60-180 km) as part of the TIMED mission. TIDI is a limb viewer and observes emissions from OI 557.7 nm and rotational lines in the O2(0-0) Atmospheric band. Wind measurement accuracies approach 3 ms-1 in the mesosphere and 15 ms-1 in the thermosphere. The TIDI instrument’s performance during the first year and a half of operation is discussed in this paper. Many subsystems are working as designed. The thermal control system is holding the instrument temperatures at their desired set-points. The CCD detector is working as expected with no changes observed in the gain, bias or read noise. The instrument suffers from a light leak that causes the background to be elevated and increases the uncertainty in the wind measurement. Nothing can be done to eliminate this problem but modeling of the background has eliminated any systematic effect. Water outgassing from the spacecraft or instrument has deposited as ice on some part of the optics and reduced the instrument’s sensitivity. This problem has been reduced by two spacecraft rolls which pointed the TIDI radiator to view more of the earth causing the optics to warm up and sublimate much of the ice.
Design and characterization of the Japanese Advanced Meteorological Imager (JAMI)
Jeffery J. Puschell, Howard A. Lowe, James W. Jeter, et al.
The Japanese Advanced Meteorological Imager (JAMI) was developed by Raytheon and delivered to Space Systems/Loral as the Imager Subsystem for the Japanese MTSAT-1R system. Detailed characterization tests show JAMI meets all MTSAT-1R requirements with margin. JAMI introduces the next generation of operational weather imagers in geosynchronous Earth orbit (GEO) and provides much improved spatial sampling, radiometric sensitivity, Earth coverage and 24-hour observation capability compared with current GEO imagers.
Miniature thermal emission spectrometer for the Mars Exploration Rover
Steven H. Silverman, Richard J. Peralta, Philip Christensen, et al.
This paper describes some details of the results of the calibration of the Miniature Thermal Emission Spectrometer (Mini-TES) being built by Raytheon Santa Barbara Remote Sensing (SBRS) under contract to Arizona State University (ASU). This paper also serves as an update to an earlier paper (Peralta, et al, 2001) for mission description and instrument design. Mini-TES is a single detector Fourier Transform Spectrometer (FTS), covering the spectral range 5- 29 microns (μm) at 10 cm-1 spectral resolution. Launched in June 2003, one Mini-TES instrument will fly to Mars aboard each of the two missions of NASA’s Mars Exploration Rover Project (MER), named Spirit and Opportunity. Mini-TES is designed to provide a key minerological remote sensing component of the MER mission, which includes several other science instruments. The first Mini-TES unit was required to meet a two-year development schedule with proven, flight-tested instrumentation. Therefore, SBRS designed Mini-TES based on proven heritage from the successful Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES), which was launched in 1996 and is still operational with over 500 million spectra collected to date. Mini-TES design, performance, integration onto the rovers, as well as details of the calibration are discussed. Full instrument and calibration details are the subject of an upcoming Journal of Geophysical Research Mini-TES paper by Christensen, et al.
Design concept for a Landsat-class imaging spectrometer with well-corrected spectral fidelity
Thomas G. Chrien, Lacy G. Cook
This paper describes a design concept for a Landsat-class imaging spectrometer. The challenge is to match the Landsat data parameters, including a 185 kilometer swath and a 30 meter ground sample distance (GSD) from a 705 km sun-synchronous orbit with a sensor which has contiguous spectral coverage of the solar reflected spectrum (400 to 2500 nanometers). The result is a dual purpose remote sensing satellite that provides global access imaging spectrometer data sets as well as fulfilling the needs of the Landsat Data Continuity Mission. Key features of the design include (1) high signal-to-noise ratio, (2) well corrected spectral fidelity across a 6000 pixel pushbroom field-of-view, (3) real-time simulation of Thematic Mapper bands 1-5, and 7 for direct continuous downlink and (4) straightforward calibration of the data to units of absolute spectral radiance.
Radiative Transfer and Geophysical Information Retrieval
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Exploitation of MODTRAN4 capabilities to predict at-sensor radiance
Top-of-atmosphere radiance is computed between 350 and 2500 nm for atmospheres containing one of three aerosol models (rural, maritime and dust) inherent to MODTRAN, over different surface reflectance values, and compared with those computed using a model of the same aerosol species derived from measurements by a global network of ground-based radiometers (AERONET). It is observed that even over high reflectance targets (R=0.5), care must be taken in the prescription of aerosol optical properties so as to limit uncertainty resulting from aerosols in the top-of-atmosphere radiance to less than 2%. It is found that, for grass and desert sites, using a simple power law exponent derived from measured spectral optical depths reduces uncertainty in the computed satellite radiance resulting from prescription of aerosol properties to less than approximately 1.5% for the aerosol species examined. Uncertainty in the computed top-of-atmosphere radiance during vicarious sensor calibration over desert sites that may result from this simple prescription of the aerosol size distribution is thus less than uncertainty in the TOA radiance resulting from measurements of the site reflectance. The new aerosol and multiple scattering capabilities of the most recent version of MODTRAN have made such studies possible and are promising for attempts to use MODTRAN in the vicarious calibration of airborne and spaceborne sensors.
The tomography algorithm for ozone retrieval from the NPOESS OMPS limb measurements
Boris Z. Petrenko, Jack C. Larsen, Arya Akmal
The Ozone Mapping and Profiler Suite (OMPS), which is a component of the National Polar-Orbiting Environmental Satellite System (NPOESS), will provide high-resolution vertical ozone profiles in the atmosphere from limb measurements of scattered solar radiation retrieved from limb-geometry measurements of scattered solar radiation in the ultraviolet, visible and near-infrared spectral ranges. Due to the_long path of scattered limb radiation through the atmosphere, ozone variations along the line of sight can have a significant impact on the measured radiance and, if not accounted for, result in significant deterioration of retrieval accuracy. In this paper we describe a tomographic retrieval algorithm, which accounts for the effects of horizontal ozone variations on the target ozone profile retrieval by restoring a fragment of the 2D ozone distribution in the neighborhood of the target profile. Our simulated retrievals of realistic 2D ozone distributions demonstrates that the tomographic algorithm provides remarkably better retrieval accuracy than the baseline 1D algorithm, which neglects horizontal ozone inhomogeneities. Special measures are taken to reduce the tomographic inversion execution time. We demonstrate that acceptable execution times can be achieved if fragments of the ozone field are retrieved sequentially along a satellite orbit.
Comparison of a single-view and a double-view aerosol optical depth retrieval algorithm
We compare the results of a single-view and a double-view aerosol optical depth (AOD) retrieval algorithm applied to image pairs acquired over NASA Stennis Space Center, Mississippi. The image data were acquired by the Department of Energy's (DOE) Multispectral Thermal Imager (MTI), a pushbroom satellite imager with 15 bands from the visible to the thermal infrared. MTI has the ability to acquire imagery in pairs in which the first image is a near-nadir view and the second image is off-nadir with a zenith angle of approximately 60°. A total of 15 image pairs were used in the analysis. For a given image pair, AOD retrieval is performed twice---once using a single-view algorithm applied to the near-nadir image, then again using a double-view algorithm. Errors for both retrievals are computed by comparing the results to AERONET AOD measurements obtained at the same time and place. The single-view algorithm showed an RMS error about the mean of 0.076 in AOD units, whereas the double-view algorithm showed a modest improvement with an RMS error of 0.06. The single-view errors show a positive bias which is presumed to be a result of the empirical relationship used to determine ground reflectance in the visible. A plot of AOD error of the double-view algorithm versus time shows a noticeable trend which is interpreted to be a calibration drift. When this trend is removed, the RMS error of the double-view algorithm drops to 0.030. The single-view algorithm qualitatively appears to perform better during the spring and summer whereas the double-view algorithm seems to be less sensitive to season.
Data inversion for the Multi-Order Solar Extreme-Ultraviolet Spectrograph
J. Lewis Fox, Charles C. Kankelborg, Tomas R. Metcalf
The Multi-Order Solar Extreme Ultraviolet Spectrograph (MOSES) is a high resolution, slitless imaging spectrometer that will observe the Sun in extreme ultraviolet near 304A. MOSES will fly on a NASA sounding rocket launch in spring 2004. The instrument records spatial and spectral information into images at three spectral orders. To recover the source spectrum, an ill-posed inversion must be performed on these data. We will explore two of the techniques by which this may be accomplished: Fourier backprojection and Pixons, constrained by the spatially integrated spectrum of the Sun. Both methods produce good results, including doppler shifts measured to 1/3-pixel accuracy. The Pixon code better reproduces the line widths.
Instrument Prototype Development for Enabling Future Space-based Observation Capabilities
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A dual-etalon imaging Fabry-Perot interferometer for observing tropospheric ozone: airborne instrument design
William B. Cook, Allen M. Larar, Carl S. Mills, et al.
Space-based observation of tropospheric trace species has been identified as a high-priority atmospheric science goal. In particular, global and regional measurements of lower atmosphere ozone concentrations are critical to both enhancing scientific understanding and to expanding capabilities for pollution monitoring. The interferometer addressed here will be a spatially imaging, spectrally tunable airborne sensor focused on making such important tropospheric ozone measurements, and is designed to be a risk-reduction and proof-of-concept test-bed for developing the corresponding orbiting instrument also based upon a dual etalon Fabry-Perot interferometer. We present herein details of the airborne instrument design and development process, including parameter specifications for the interferometer and other enabling subsystems, as well as plans for integration, test, and characterization in the laboratory.
Far-infrared spectroscopy of the troposphere (FIRST): sensor development and performance drivers
Gail E. Bingham, Harri M. Latvakoski, Stanley J. Wellard, et al.
The radiative balance of the troposphere, and hence global climate, is dominated by the infrared absorption and emission of water vapor, particularly at far-infrared (far-IR) wavelengths from 15-50 μm. Water vapor is the principle absorber and emitter in this region. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are widely recognized as major uncertainties in our understanding of current and the prediction of future climate. Cirrus clouds modulate the outgoing longwave radiation (OLR) in the far-IR, and up to half of the OLR from the Earth occurs beyond 15.4 μm (650 cm-1). Current and planned operational and research satellites observe the mid-infrared to only about 15.4 μm, leaving space or airborne spectral measurement of the far-IR region unsupported. NASA is now developing the technology required to make regular far-IR measurements of the Earth’s atmosphere possible. Far InfraRed Spectroscopy of the Troposphere (FIRST) is being developed for NASA’s Instrument Incubator Program under the direction of the Langley Research Center. The objective of FIRST is to provide a balloon-based demonstration of the key technologies required for a space-based sensor. We discuss the FIRST Fourier transform spectrometer system (0.6 cm-1 unapodized resolution), along with radiometric calibration techniques in the spectral range from 10 to 100 μm (1000 to 100 cm-1). FIRST will incorporate a broad bandpass beamsplitter, a cooled (~180 K) high throughput optical system, and an image type detector system. The FIRST performance goal is a NEΔT of 0.2 K from 10 to 100 μm.
SIRAS-G, the Spaceborne Infrared Atmospheric Sounder for Geosynchronous Earth orbit: a versatile concept for IR imaging spectrometry from geosynchronous orbit
The Spaceborne Atmospheric Infrared Sounder for Geosynchronous Earth Orbit (SIRAS-G) represents a new approach to infrared imaging spectrometry suitable for Earth observation from geosynchronous orbit. SIRAS-G, selected for development under NASA’s 2002 Instrument Incubator Program, requires less mass and power than heritage instruments while offering enhanced capabilities for measuring atmospheric temperature, water vapor profiles, and trace gas column abundances in a compact package. The flight instrument concept measures infrared radiation in 2048 spectral channels with a nominal spectral resolution (Δλ/λ) of 1100. Combined with large 2-D focal planes, this system provides simultaneous spectral and high-resolution spatial imaging. In 1999, the SIRAS team built and tested the LWIR (12.0 - 15.4 μm) spectrometer under NASA’s Instrument Incubator Program (IIP-1999). SIRAS-G builds on this experience with a goal of producing and demonstrating the performance of a laboratory demonstration instrument. In this paper, we describe planned development activities and potential future scientific instrument applications.
New Instrument Concepts for Future Space-based Remote Sensing Systems
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Imaging Multi-Order Fabry-Perot Spectrometer (IMOFPS) for spaceborne measurements of CO
An instrument concept for an Imaging Multi-Order Fabry-Perot Spectrometer (IMOFPS) has been developed for measuring tropospheric carbon monoxide (CO) from space. The concept is based upon a correlation technique similar in nature to multi-order Fabry-Perot (FP) interferometer or gas filter radiometer techniques, which simultaneously measure atmospheric emission from several infrared vibration-rotation lines of CO. Correlation techniques provide a multiplex advantage for increased throughput, high spectral resolution and selectivity necessary for profiling tropospheric CO. Use of unconventional multilayer interference filter designs leads to improvement in CO spectral line correlation compared with the traditional FP multi-order technique, approaching the theoretical performance of gas filter correlation radiometry. In this implementation, however, the gas cell is replaced with a simple, robust solid interference filter. In addition to measuring CO, the correlation filter technique can be applied to measurements of other important gases such as carbon dioxide, nitrous oxide and methane. Imaging the scene onto a 2-D detector array enables a limited range of spectral sampling owing to the field-angle dependence of the filter transmission function. An innovative anamorphic optical system provides a relatively large instrument field-of-view for imaging along the orthogonal direction across the detector array. An important advantage of the IMOFPS concept is that it is a small, low mass and high spectral resolution spectrometer having no moving parts. A small, correlation spectrometer like IMOFPS would be well suited for global observations of CO2, CO, and CH4 from low Earth or regional observations from Geostationary orbit. A prototype instrument is in development for flight demonstration on an airborne platform with potential applications to atmospheric chemistry, wild fire and biomass burning, and chemical dispersion monitoring.
Tandem-etalon tunable filter for NIR spectral imaging
John Noto, Yan Betremieux, Robert B. Kerr, et al.
Scientific Solutions Inc. (SSI) has developed a tunable liquid crystal Fabry-Perot (LCFP) etalon system comprised of a resolving and a suppression etalon in tandem. The 30-micron resonant cavity spacing of the resolving etalon provides for high spectral resolution while the system maintains the significantly broader free spectral range of the 6-micron gap suppression etalon across the tunable region. An applied electric field alters the ordinary refractive index of nematic liquid crystal cells within each etalon cavity, thereby altering the resonant properties of the etalons, allowing for system tunability over several orders of interference. This system acts as a tunable optical filter with an operating range from 700nm to 1100nm. Testing of the LCFP etalon system with both a high resolution Czerny-Turner monochrometer and a stabilized ND:Yag laser demonstrate a FWHM of 0.67nm to 1.03nm. System transmission reaching 70% of polarized light is achieved with tunability over one free spectral range in approximately 30 milliseconds. The free spectral range of the tandem etalon system ranges from 27nm-36nm over the operating range, and allows for 40 randomly selectable spectral channels per free spectral range. This system is designed for use in spectral imaging systems, initially for the semiconductor industry, but is equally applicable to the earth remote sensing community.
Remote Sensing Technologies
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Lossy data compression for imaging interferometer data using a wavelet-transform-based image compression algorithm
Data compression on future space-based imaging interferometers can be used to reduce high telemetry costs, provided the performance is acceptable. This paper investigates lossy data compression of imaging interferometer datacubes using a wavelet transform-based compression algorithm, the Set Partitioning in Hierarchical Trees (SPIHT) image compression algorithm. Compression is performed on individual frames of the interferogram datacubes. Simulated datacubes from the Geosynchronous Imaging Fourier Transform Spectrometer (GIFTS) are modified to produce new complex GIFTS datacubes used to perform the experiments. Separate programs are written for the encoder and decoder in C++. The encoder and decoder are simulated to the bit-level, meaning they simulate the exact bit streams that would be generated by hardware implementations. All compression ratios reported are based on the actual file size of the encoded data. The simulations indicate very high performance of the algorithm in the interferogram domain, with average errors of less than one least significant bit (LSB) for the GIFTS long-wave band and just over one LSB for the GIFTS short/mid-wave band at compression ratios as high as 13.7:1 and 15.4:1, respectively. At the same compression ratios, errors in the spectral radiance domain are comparable to the simulated instrument noise and RMS temperature profile retrieval errors of less than 1 K are achieved using a University of Wisconsin-Madison prototype retrieval algorithm.
Optimization strategy for low-phase-dispersion dielectric mirrors
The Damped Least Squares (DLS) optimization algorithm plays a useful role in the design of multilayer optical coatings. It does, however, require numerically well-behaved error terms and the convergence is highly dependent on the degree of non-linearity present in the error term chosen. This paper makes some recommendations for the choice of error terms for effective control of phase-dispersion in dielectric mirrors.
Poster Session
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Experimental data on CO2 detection using Fabry-Perot-based optical setup for atmospheric observations
Elena Georgieva, Emily Wilson, Mariusz Miodek, et al.
The experimental data on CO2 detection in atmosphere using Fabry-Perot technique are presented. The optical setup consists of two channels. Channel one measure the total reflected light, whereas channel two uses a solid substrate Fabry-Perot etalon to restrict measurement to light in CO2 absorption bands. The free spectral range of the etalon is calculated to be equal to the almost regular spacing between the CO2 spectral bands located near 1.58 μm, where CO2 absorption is significant. The ratio of the intensities detected by the two channels is then sensitive to CO2 change. We are exploring the temperature dependence of the index of refraction of the optical media to align the pass bands of the Fabry-Perot etalon to the appropriate CO2 absorption lines. The experimental data presented show excellent agreement with our theoretical expectations. They are recorded at different gas pressures and temperatures. Some of the major advantages of the optical setup are its compactness, high sensitivity, high signal-to-noise ratio, and stability.
Irradiance calculation over mountainous areas in the reflective spectral domain: comparison with an accurate radiative transfer code
A new model has been developed to estimate irradiance at ground level over a rugged terrain in the reflective spectral domain in order to be used in an hyperspectral inversion code. Modtran4 allows to calculate atmospheric parameters over a flat scene which are then used to estimate the four components of irradiance over a mountainous area (direct, diffuse, reflected and coupling irradiance). This method have been compared with an accurate radiative transfer code called AMARTIS. Simulations are done at three wavelengths and for two solar configurations over a relief composed of two hills and flat terrain. Irradiances obtained with our model are in good agreement with this reference code except in shadow areas in the SWIR. Our model is also compared with a currently used model developed by Sandmeier whose results are worse than our model's results. Current relative errors of our diffuse, reflected and coupling irradiance calculation model do not have much influence on total irradiance in most of the cases. This influence become significant for high beam incidence angles where Digital Elevation Model errors can be much more important.
The High-Resolution Doppler Imager: status update 12 years after launch
Wilbert R. Skinner, Alan R. Marshall, David A. Gell, et al.
The High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) has been measuring winds in the stratosphere, mesosphere and lower thermosphere since November, 1991. The winds are determined by measuring the Doppler shift of emission and absorption lines in the O2 Atmospheric Band that are located between 630 and 762 nm. HRDI is a triple-etalon Fabry-Perot interferometer that has a resolution of ~0.05 cm-1 and very good white light rejection. A multi-channel detector with 31 channels is used to examine a spectral region 0.5 cm-1 wide and an adjustable filter wheel permits the selection of any one of 13 spectral bands. The long life of this instrument has presented many challenges in keeping the calibrations current and in compensating for inevitable degradations in instrument and spacecraft performance. Some of the problems with the UARS spacecraft the affect HRDI operations are: limited power due to the solar array drive failure; loss of data resulting from a failure of the tape recorders, and loss of attitude knowledge caused by the failure of the star trackers. HRDI has shown little loss in capability over the years with only a decrease in the azimuth rate of the telescope motor a significant sign of aging. This paper will discuss some of these challenges and how they have been met.
Cloud-clearing algorithm error vs. sensor footprint size and configuration with optional multispectral MODIS imagery
We provide an evaluation of cloud clearing error and sensor footprint size in high spectral resolution sounding instrumentation. Input data are actual atmospheric spectra collected by the NPOESS Aircraft Sounding Testbed - Interferometer (NAST-I) [see Cousins and Smith (1998), and Smith et. al., (1999, 2001)]. NAST-I data is averaged to create sensor configurations of varying field of view size and array number. The cloud-clearing techniques, based on the N* approach (Smith, 1968, Chahine, 1977, and McMillin and Dean, 1982), use the linear relation between observed radiance and cloud amount to extrapolate the radiance observed for two adjacent fields of view possessing differing cloud amounts to the cloud free value. The option of including MODerate resolution Imaging Spectroradiometer (MODIS) style data was also investigated. With the MODIS filter, the assumption of cloud emissivity homogeneity is not needed because of the MODIS high spatial resolution spectral channels in which the clear air radiance can be defined for a scene. This relaxation of the need for cloud optical property homogeneity enables a higher yield of valid clear column radiance estimates. We show that the use of MODIS-like multi-spectral imagery data in the cloud clearing of high spectral resolution sounder data will minimize the dependence of the sounding retrieval accuracy and yield on instrument field-of-view size. The errors of the multi-spectral MODIS cloud-cleared spectral radiance are generally a factor of two lower than those errors associated with the use of a single window channel for the cloud clearing of radiance spectra.